Apparatus and method for efficient scheduling of tasks

ABSTRACT

A system and method of scheduling tasks, comprising receiving activity and performance data from registers or storage locations maintained by hardware and an operating system; storing calibration coefficients associated with the activity and performance data; computing an energy dissipation rate based on at least the activity and performance data; and scheduling tasks under the operating system based on the computed energy dissipation rate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No. 12/841,154, filed Jul. 21, 2010, issued Mar. 12, 2013 as U.S. Pat. No. 8,397,088, which is a non-provisional of U.S. Provisional Application No. 61/227,361, filed Jul. 21, 2009, each of which is expressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of server energy and cooling management.

BACKGROUND OF THE INVENTION

The data center energy crisis has been in the making for the past several decades, as data centers are designed primarily with peak performance and peak capacity in mind. With the doubling of transistor counts and performance in semiconductor devices at 18-month intervals following Moore's law, energy dissipation in servers have grown at an alarming rate. The smaller form factors of modern blade servers have, at the same time, permitted more and more servers to be packed into a given physical space, further worsening the already critical situation with server power dissipations within data centers. Adding to all of this is the trend to overprovision data center capacities and the use of overrated power supplies for the individual servers. Such over provisioning results in gross energy inefficiencies as servers and power supplies are generally designed to give very high energy efficiencies only at or near peak loading levels. The net result of all of these is that 50% and upwards of the total cost of ownership (TCO) for a data center is in the utility costs of operating and cooling the servers. From an economic standpoint, we spend about 2% of the nation's annual energy consumption on data centers. With electricity costs growing annually at about 7% to 10%, the situation is bleak and needs immediate correction with the use of innovative and dramatic solutions. The other benefits of operating energy-efficient data centers are of no less significance—reducing the carbon footprint and making the nation energy-secure are also worthy goals.

Traditional approaches to managing the data center energy crisis have been to use advanced cooling and packaging solutions, to use DC power sources for servers and a variety of other solutions at reducing the energy dissipation within servers. These latter solutions have included the use of dynamically changing the power-performance settings for individual server components, such as processors and hard disk drives, or on policy-based job scheduling that schedule the offered workload across servers to meet thermal objectives. The growing use of virtualization technologies in data center also supports flexible scheduling based energy management schemes. Virtually, all of these solutions are reactive in nature: energy management or cooling solutions are adjusted based on the feedback from sensors that sense temperature or some activity parameter (such as current computing load, performance metrics).

SUMMARY OF THE INVENTION

The present technology assumes, according to one embodiment, a holistic view of data centers as a cyberphysical system where the cooling solutions work in unison with the computing level solutions for energy management in a coordinated fashion. The total energy expended in the computing components and the energy expended in the cooling system is treated as a first class resource that needs to be scheduled explicitly to maximize the overall energy-efficiency of the data center. One embodiment of aspects of the technology is multi-tiered and includes:

-   -   The use of fast models for predicting local and global thermal         conditions to promote overall energy efficiency. The thermal         models, in turn, are driven by empirical models of energy         dissipation within servers and switches as a function of the         measured values of a variety of actual activity counts. This         approach of jointly using accurate energy dissipation models for         the computing equipment and fast thermal models permit the         cooling solutions (adjustment of inlet temperature, air flow         speed and pattern) to be proactive.     -   The use of a global scheduler to allocate individual energy         budgets to servers as a function of the workload, the predicted         thermal trend, actual server utilizations and temperature and         airflow measurements from sensors. The cooling efforts are also         matched to the predicted thermal trends and are rack specific,         instead of being rack agnostic, as in traditional systems.         Alternatively stated, the cooling efforts for a rack are         directed, dynamic and matched to the thermal conditions in the         rack's environment. This results in the most energy-efficient         use of the cooling resources.     -   The use of modified server operating system kernels that permit         the individual servers to stay within their assigned energy         consumption budget. Software solutions at the operating system         kernel level exercise existing power management actuators inside         the processor and other components of servers in a proactive         fashion to stay within the dictated energy budget and in a         reactive fashion based on the thermal condition of its         environment. Thus, the system uses a predictive model of the         thermal conditions based on analysis of a set of “tasks” or         other prospective activities, as well as a feedback driven         control which employs sensors or indicia or actual conditions.         The predictive model may be adaptive, that is, the predictive         model may be modified in dependence on the actual outcomes as         determined by the sensors or indicia. In addition to the sensor         or indicia inputs, the system may also receive a price or cost         input, which permits a price or cost optimization, rather than         an efficiency optimization. By imposing an external price or         cost consideration, the system can be made responsive to peak         energy demand considerations, and also a prioritization of         tasks, which may each be associated with a task value.

Each of these technologies may be employed together, separately, or in subcombination. The thermal models, for example, can be implemented with minor modification to semiconductor devices, to provide software access to registers and counters which monitor operation of the chip. As the chip processes information, various types of activities are tracked, and these tracked activities may then be read by software to implement the models. The models may be executed on the same semiconductor as an additional process within a multitasking processing stream, within a special core dedicated to this process, either on or off the integrated circuit, or by a remote system. The modified server operating system kernels typically do not require hardware modifications, though sensors may be required beyond those present in standard components of the computing system. In particular, integration and interfacing of external cooling system sensors and controls may require additional hardware modules. The global scheduler is typically provided as part of a load distribution switch, which is a standard hardware component, but executes software in accordance with the present embodiments. In particular, the task allocation algorithm favors loading of servers to near capacity, which may be defined by performance limitations or thermal limitations, before allocating tasks to other servers. The allocation may distinguish between different blades within a rack, with each rack typically being controlled on a thermal basis, i.e., to stay within a desired thermal envelope while achieving cost-efficient cooling, while each blade may be allocated tasks which balance performance and energy efficiency, while remaining within safe thermal limits.

The net result of a combination of all of this is a control system that uses a combination of proactive and reactive elements in a multi-tiered strategy for co-managing the thermal and computing solutions for promoting the energy efficiency (or cost effectiveness) of the data center. However, these technologies need not be employed together to gain benefits. Likewise, the chip, operating system (software), and system level optimizers need not communicate with each other, though they are preferably aware of the multilevel optimizations, which may alter responses to conditions. For example, a prediction of and control over future processing load must be coordinated between the various system levels in order to avoid conflicting efforts or over-compensation.

A preferred embodiment may be implemented in a scaled down data center consisting of Linux server racks with floor plenum and portable computer room air conditioners (CRACs) and a variety of sensors, or a full data center with server racks in a facility with centrally or distributed control cooling system. Preliminary results indicate that the present approach can realize about a 20% improvement in the energy efficiency of the data center.

Efficient scheduling and power management techniques for techniques can utilize an accurate estimate of the energy dissipated by the entire computing system, e.g., a server blade, comprising of the microprocessor, chipset, memory devices, peripheral controllers and the peripherals. Facilities are provided for estimating the total energy consumption of the system in a given time period, and exposing that estimate to software via registers or special ports or storage locations. The measurements made are specific to the actual platform configuration. Thus, by correlating characteristics of code executing on the system, with the facilities, and correlating the facilities with energy consumption and/or thermal dissipation and/or other thermal parameter, a system can then predict the energy consumption or thermal results of executing various code. The system may execute various tasks concurrently, and the system response may be dependent on the interaction of the tasks; therefore the system, when seeking to schedule or allocate a new task to a processing queue, considers characteristics of the existing task load, and the anticipated incremental change in system state(s) as a result of adding the task of the computing environment. In general, the task will have an incremental effect on the facilities, though in some cases an interaction between tasks makes the combination non-linear. In any case, a model is employed to determine the effect of a proposed action with respect to a new task, e.g., executing immediately, delaying execution, trading the task with another queue, etc., on the facilities. The predicted future facilities are then analyzed to determine the change in efficiency (e.g., Watts per unit value output), energy consumption, or relevant thermal parameter. The control module then applies its optimization criteria to control task execution. For example, if the optimization criteria is maximum energy efficiency with no more than 25% performance degradation vs. a traditional load balancing server environment (though degradation is not necessarily a result of the process), the control will seek to achieve these criteria with existing resources. If the criteria cannot be met, then the task may be rejected from the queue, the result of which may be reallocation of the task to another system, which may result in recruitment of inactive servers to active status.

Because the optimization is at the level of a single system, significant consideration and processing of the available data may be incurred. As necessary, additional processing capacity may be added to the system, typically in the form of an additional processing core or CPLD, to perform the optimization; however, it is anticipated that the optimization load will be a small portion of the system processing capacity, and that the processing will be performed using a normal CPU of the system.

The optimization may, in some cases, be used to alter a system performance setting. For example, a processor clock speed may be dynamically changed in dependence on load. This adaptive clock speed may be responsive to the optimizer, and thus need not respond directly to the processing load, especially if this would result in loss of efficiency, especially if certain performance degradation is acceptable.

Preferably, the optimization can be transparent to external systems, thus permitting this technology to be “stand alone”. Of course, there can be communications protocols with other compatible elements of the infrastructure, such as other systems, rack level controls, and room level controls, in order to coordinate actions.

The preferred physical location of this facility for computing the energy dissipation is within the processor, but it could be located within the chipset or implemented in a distributed fashion over several physical components of the system.

According to one aspect, a processor or unit computing system maintains a set of registers which respectively record a type of activity. The activity recorded by the registers is preferably predictable from the tasks allocated to the processor or unit. Therefore, a predictive model may be used to correlate an extrinsic variable with task characteristics. In particular, the extrinsic variable may be a thermal variable, such as temperature, power dissipation, or perhaps a derivative such as temperature rise rate, local temperature differentials (e.g., between different areas of the processor or unit). The correlation may be empirically corrected, and thus individual system variation can be compensated. Likewise, for software modules which are repetitive, the response of a system to a particular sequence of instructions or program module may be predicted based on past performance, even if the register-based calculations themselves are less than completely accurate. For example, in some cases, a processor may have variable voltage or clock rate, which is dependent indirectly on the current instructions. The registers which track activity may not compensate for system changes, and even if they do compensate, the compensation itself must be considered in the optimization.

It is therefore an object to provide a system for, and method for scheduling tasks, comprising: receiving at least one of activity and performance data from registers or storage locations maintained by a programmable hardware system; retrieving stored calibration coefficients associated with the activity or performance data; computing an estimate of energy dissipation within a given time interval based on at least the activity and performance data; and at least one of scheduling tasks for, and adjusting an energy dissipation characteristic of, at least one processing core, based on the computed energy dissipation.

The data may be received from registers or storage locations is both activity data and performance data.

The activity data and performance data may be generated by hardware counter registers associated with operation of a processor core, which generate an interrupt upon overflowing, and are readable by software executing on the processor core.

The calibration coefficients may be derived empirically from an analysis of energy dissipation with respect to time interval for execution of software tasks on the programmable hardware system.

The activity data may comprise at least cache misses, and performance data may comprise at least instruction processing completions.

The registers or storage locations may be collocated on an integrated circuit with the hardware whose activity or performance data is being maintained, and are updated by dedicated hardware. The registers or storage locations may also be located remotely with respect to the hardware whose activity or performance data is being maintained.

The registers or storage locations may be updated under control of software executing on a programmable processor.

The computing and the at least one of scheduling and adjusting may be performed by software executing on a general purpose processor, which reads hardware registers under control of the software. The at least one of scheduling and adjusting may also be performed under control of dedicated hardware processing elements.

It is a further object to provide a system for, and method, comprising, comprising: executing a plurality of tasks in a processing system, each using a plurality of processing resources, each resource being associated with an energy consumption; monitoring at least one of activity and performance data in a set of registers maintained by the system during execution of the plurality of tasks; monitoring an actual energy consumption over a time interval of the system while performing each of the plurality of tasks; deriving an algorithm that predicts an energy consumption characteristic associated with each task attributable to each respective resource, based on the data from the set of registers and the monitoring, to calibrate the data from the set of registers to provide an accurate indication of the energy consumption characteristic; and at least one of scheduling a new task for execution by, and adjusting an energy dissipation characteristic of at least one component of, the processing system, in dependence on at least a state of the processing system and the algorithm.

The data may be received from registers or storage locations is both activity data and performance data.

The set of registers may comprise hardware counter registers associated with operation of a processor core, which generate an interrupt upon overflowing, and which are readable by software executing on the processor core.

The system may monitor activity data comprising at least cache misses, and monitor performance data comprising at least instruction processing completions.

The set of registers may be collocated on an integrated circuit with at least one processing resource, the set of registers being updated by dedicated hardware.

The set of registers may also be located remotely with respect to at least one processing resource whose at least one of activity and performance are being monitored.

The set of registers may be updated under control of software executing on the processing system.

The at least one of scheduling and adjusting may be performed by software executing the processing system, which reads the set of registers under control of the software. The at least one of scheduling and adjusting may also be performed under control of dedicated hardware processing elements.

It is another object to provide an apparatus, comprising: a first memory configured to store data relating to at least one of activity and performance of components of a computing system which is updated concurrently over a course of time; a second memory configured to store calibration coefficients associated with the data; a processor, configured to execute an algorithm based on at least the data stored in the first memory and coefficients stored in the second memory, to compute an output corresponding to an estimate of energy dissipation of the computing system in a given time interval; and an interface, configured to communicate the computed energy dissipation of the computing system in the given time interval to at least one of an activity scheduler and a computing system performance adjuster.

The data may be both activity data and performance data. The activity data and performance data may be generated by hardware counter registers associated with operation of a processor core of the processor, which generate an interrupt upon overflowing, and are readable by software executing on the processor core, the activity data comprising at least cache misses, and the performance data comprising at least instruction processing completions.

The calibration coefficients may be derived empirically from an analysis of energy dissipation with respect to time interval for execution of software tasks on the computing system.

The first memory may be collocated on an integrated circuit with at least one component, and be updated by at least dedicated hardware.

The first memory may be updated under control of software executing on the computing system and the at least one of the activity scheduler and the computing system performance adjuster are implemented in software on the computing system. The first memory may also be updated under control of dedicated hardware and the activity scheduler is controlled by dedicated hardware.

The at least one of activity and performance of components of the computing system may comprise at least one of: an overall number of instructions committed; a number of types of instructions that are committed within each processing core, a measurements of memory activity, a number of instructions that are fetched but not committed, a metric of cache coherency maintenance activity, an input/output activity quantitative and qualitative characterization, a quantitative utilization of a core, and a performance setting of a core, and a specialized coprocessor metric.

The time interval is, for example, shorter than a duration between two consecutive times at which at least one of a system performance and a system energy dissipation control signal can change. For example, the interval is shorter that a processor core performance adjustment interval, e.g., voltage and/or frequency, such as Speedstep.

The interface may produce a signal which is configured to at least one of maintain an allocated energy budget for a given interval, and maintain an operating temperature within a safe operating temperature limit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the control system aspects of the present data center management strategy.

FIG. 2A depicts the state of affairs in prior art servers and shows how the power dissipation and energy efficiency of a typical server varies with server utilization.

FIG. 2B depicts the intended overall impact of the present solution on server power dissipation and server energy efficiency plotted against server utilization.

FIG. 3 shows a block diagram of a prior art computing system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a prototype embodiment, a scaled down data center is provided which demonstrates a unique approach to addressing the data center energy crisis. The energy spent on the computing equipment and by the cooling system is treated as a first class resource and managed explicitly in the present approach in a proactive as well as reactive manner. Instead of the traditional approach of cooling the server racks uniformly, dynamic and directed cooling is employed, that skews the cooling efforts to match the actual and projected cooling demands of the individual or groups of racks. Cooling for a rack is controlled based on sensors (i.e., a reactive control), a prospective set of tasks or functions in a queue (i.e., a proactive control), and an operating system component of each subsystem which permits a modification of energy demand.

It is noted that a cooling system may have higher efficiency when cooling a relatively hotter server than a cooler one, and therefore overall efficiency may be increased by permitting some server racks to run near a maximum operating temperature, and other racks to be essentially deactivated, pending peak demand recruitment. While running at relatively higher temperatures may be a factor in reducing a mean time between failures (MBTF), the usable life of blades in a data center is typically well in excess of the economic life; further, even if there is a failure, the data center will typically have automatic failover fault tolerance systems. Indeed, if some racks in the data center are specifically designed to always run near peak capacity and high temperature, these may be configured for more efficient operation, for example, greater spacing from other racks, to permit better heat load shedding without affecting adjacent racks, and higher temperature specification components.

It is also noted that in some cases, it is not the temperature per se which adversely impacts the MBTF of a system, but rather the thermal cycling and mechanical stresses on components, circuit boards, and packaging. In such cases, the operation of a rack at a consistent hot temperature may be an advantage over a system which seeks, for example, a uniform minimum temperature of all racks which varies with data center load.

One embodiment of the technology improves the overall energy-efficiency of a data center in a holistic manner, and targets both the energy expended in operating the equipment and the energy expended in the cooling system. A key aspect of is to coordinate the activities of all of the energy consumers in a data center. These consumers include the individual severs and communication infrastructures as well as the cooling system components. Some current solutions to this problem have addressed inefficiencies in the use of power conversion devices, the cooling system and the servers themselves [Sh 09, BH 07, BH 09, LRC+ 08]. Emerging solutions to this problem have also started to address the need to coordinate the activities of these consumers [BH 09, NSSJ 09, SBP+ 05, TGV 08]. As an example, the work of [TGV 08] has proposed an approach for minimizing the energy expended on the cooling equipment by minimizing the inlet temperature through appropriate job scheduling. The work of [NSSJ 09] coordinates the energy expended on the computing equipment and the cooling infrastructures and allocates energy budgets to virtual machines. Such VM energy budgets are not easy to implement, as energy expended by a VM is not easy to track and control; energy dissipation in many related components are ignored in simplifications that are used. In general, emerging solutions have a number of potential limitations:

-   -   The energy and performance overhead associated with job         rescheduling and VM management and server-local scheduling         overhead are ignored. The communication infrastructures within a         data center are heavily utilized and are prone to congestion,         resulting in significant added energy dissipation if jobs are         rescheduled.     -   A simple rescheduling of the jobs may not make the most         energy-efficient use of the servers and racks—the operating         configurations of such servers have to be continuously adapted         to fit the characteristics of the workload.     -   Simple reactive control systems, as proposed in all existing and         emerging solutions, do not address the problem of thermal lags         and delays associated with temperature sensors, whose inputs are         used by the actuators in these systems.     -   The implicit assumption in most current systems that that all         servers and racks have a uniform external cooling requirement         may not be the best one for improving overall energy efficiency.         While we do have some proportional cooling facilities in the         form of automatically adjusted CPU cooling fan and enclosure fan         speeds, external cooling systems are generally uniform and         oblivious of the specific cooling needs of an entire rack. In         general, higher energy efficiency will result by redirecting         additional cooling to regions that can benefit from it,         resulting in a dynamic, directed cooling system.

The present approach allocates energy budgets to servers, racks, storage and communication components and adapts the cooling effort dynamically to match the energy dissipated in these components. The energy consumption in the computing components are modeled using accurate empirical formulas and server-local (and global) scheduling techniques are used to limit server energy consumption within the allocated budget. This is a far more practical approach compared to any scheme that operates on the basis of energy budget allocations to VMs. The energy dissipation estimates from these empirical models are used to schedule the energy budgets for the computing equipment and the dynamic cooling system, along with the workload. Last but not the least, the present control system uses both proactive and reactive control mechanisms to manage the data center effectively in the face of sudden workload variations and to mitigate latencies associated with the activation and deactivation of servers and VMs.

In current data centers, the software systems infrastructures (including the Linux OS and popular file systems) are very limited in their adaptation capabilities in this respect. The most popular mechanism used for adaption is dynamic voltage and frequency scaling (DVFS) on the processing cores, and other components of the computing platform are unaddressed. This is not a desirable situation from the standpoint of energy efficiency, as the total of the energy dissipations within the DRAM modules and in the backplane and other communication infrastructures is about 45% of the total energy expended by a server, while the processors consume about 30% of the total energy [BH 09]. Current measurements seem to indicate that the processor energy dissipation will continue to decrease relative to the energy dissipation within the other components of a server [BH 09]. At the server level, it is thus critical to incorporate mechanisms that address the energy dissipation across all major components of a server instead of just focusing on the processing cores.

At the data center level, the energy expended in the communication infrastructures (switches, in particular) and in the cooling system itself should be considered. The present approach considers the total energy expended in the computing, storage, communications and cooling system as an explicitly scheduled resource and to schedule the computing and cooling resources using a common framework. The end goal is to maximize the energy efficiency of the data center, consistent with the performance goals. As discussed above, a cost optimization paradigm may also be implemented. In a cost optimization, the costs and benefits are normalized, and a set of conditions with a maximum net benefit is selected. The costs in this case may be energy costs, though other costs can also be considered in the calculation, such as maintenance costs, operating costs, license fees, etc. The benefits are typically considered as the net work output of the system, e.g., computing results, though values may be placed on the speed, latency, accuracy and completeness, etc. of the result. Indeed, assuming the same computational task, the result may be worth more to some users than others. Thus, the energy efficiency considerations may be modified or distorted based on a variety of extrinsic factors. The cost optimization factors may be analyzed in a centralized controller, which permits an allocation of tasks at a scheduler or load balancer element, distributed to the various processing cores and made part of the modified operating system kernel, or a hybrid approach. Of course, other elements may also provide these functions.

Example Use Integrated, Dynamic Management of Computing and Cooling Resources

The system preferably makes the best use of the energy expended in operating the computing and communication equipment as well as the energy expended in the cooling system. The energy expended by the computing and communication equipment and the cooling system is considered a first class resource and managed explicitly. Servers are allocated individual energy budgets and a modified Linux kernel in the servers is used to dynamically adjust the system settings and perform a local scheduling to stay within the individual server's energy budget allocation. The computation of the energy budgets for servers/racks and the control of the cooling system to effectively define a thermal envelope (that is, cap) for each server/rack for is done by a global module that senses a variety of conditions, as described later, to direct global job scheduling and to control the cooling system components, skewing the cooling effort across racks and regions as needed to improve the overall efficiency of the cooling system.

Another distinguishing feature of a preferred embodiment of the system is in its use of three controls for adapting a cooling system: the air flow rate directed at the racks from portable CRACs, the inlet temperature and the use of movable baffles to redirect air flow. Traditional solutions have largely looked at one or two of these adaptation techniques (mostly inlet temperature and somewhat rarely, air flow rate).

Using the terminology of [RRT+ 08], the integrated data center management technique is essentially a control system with the following components critical to the management:

-   -   Sensors: On the thermal/mechanical side, the sensors monitor the         temperature and air flow rates in various parts of the rack and         the room. On the computing side, the sensors are in the form of         hardware instrumentation counters within the processing cores,         counters for device and system utilizations maintained by the         operating systems, variables that record the incoming queue size         and others.     -   Actuators: Our management policy exercises various actuators to         adapt the cooling system and the servers. On the         thermal/mechanical side, the actuators adjust fan rates for         regulating the air flow from CRACs, operate servo motors to         adjust the baffles for air flow direction and use         electromechanical subsystems to adjust the inlet temperature. On         the computing side, the software elements used as actuators (a)         control the voltage and frequency settings of the cores and         activate/deactivate individual cores to ensure that they do not         exceed their allocated energy budget and to respond to thermal         emergencies at the board/component level; (b) schedule ready         processes assigned to a server and adjust core settings (using         (a)) to maximize the energy efficiency of the server; (c)         perform global task scheduling and virtual machine activation,         migration and deactivation based on the dynamically computed         thermal envelopes and rack/server level energy budgets.     -   Controllers: The control policy itself will be comprised of two         parts; the proactive and reactive, which are described in detail         below.

FIG. 1 depicts the control system aspects of one embodiment of a data center management strategy. This control system uses a combination of proactive and reactive strategies:

Proactive strategies: two different types of dynamic proactive management of data centers are provided. These are:

1. Because of thermal lags, temperature sensors are unable to detect the onset of thermal emergencies due to sudden bursty activities with the server components, including those within the DRAM, cores, local (swap) disks, if any, and the network interfaces. Empirical power models for the server energy dissipation are therefore derived, using activity counters maintained within the Operating System and the built-in hardware instrumentation counters, as described below. The estimate of the energy dissipation of an individual server is based on sampled estimations of the activities (similar to that described in [PKG 01]). This estimate of the energy dissipated by a server within a sampling interval is used to guide local scheduling and control the local system settings. The estimates of the server energy dissipations within a rack are also used as the inputs to a fast, optimized and calibrated thermal model that provides data on the thermal trends, taking into account the environmental conditions. The computed thermal trends are used, in turn, to guide global and rack level job scheduling and VM management as well as to proactively direct cooling efforts towards a region of rising temperature/hot spot.

2. The front end queues of the switches used for load balancing are a good indicator of the offered computing load to a server. These queues are therefore monitored to proactively schedule new jobs in a manner that improves the overall energy efficiency of the data center. This proactive monitoring of the input queue also permits absorption of some of the latencies involved in activating racks and servers that are in a standby mode, as well as to absorb some of the latencies in VM migration. In fact, as described below, the proactive monitoring of the incoming queues of the load balancing switches also permits activation/deactivation and migration of VMs, taking into account the energy overhead of such management.

Server Management

The goal of our proposed effort is to improve the overall energy efficiency of the servers and the cooling system. To do this, we attempt to minimize the number of active servers and operate them at or near their peak loading level to maximize their energy efficiency. The existence of virtual machine support certainly makes this approach practical. At the same time, we minimize the energy consumption in the cooling system by just providing sufficient cooling for the active servers. FIG. 2A depicts the state of affairs in current servers and shows how the power dissipation and energy efficiency of a typical server varies with server utilization. As seen in FIG. 2A, the energy-efficiency is quite low at low server loading (utilization) and the power dissipation remains relatively high. FIG. 2A also depicts the typical operating points of servers—the typical average server loading is significantly lower than the peak loading—as a result, the overall energy efficiency is quite low at these typical operating points.

FIG. 2B depicts the intended overall impact of the present technology on server power dissipation and server energy efficiency plotted against server utilization. The present multi-tiered server power management technique (which subsumes standard power management techniques) improves the server energy efficiency dramatically and simultaneously reduces the power dissipation at lower server utilization levels. The overall server efficiency thus remains quite high at the typical load levels and across a wider range of loading, as shown in FIG. 2B. Second, by globally scheduling more work to a fewer number of active servers (and by keeping the non-active servers in a standby state), we push the workload level on individual servers more towards a region where energy-efficiency is very high. The expected result of all of this is a solution that, based on a quick back-of-the-envelope calculation, can enhance the overall energy efficiency of servers by about 15% to 25% on the average beyond what is provided by the state-of-the-art, even when the added overhead of the present solution is factored in. Improvements in power savings are expected to be similar. One down side of operating servers at or near their peak capacity is that any sudden changes in the behavior of their assigned workload can cause switching activities to go up and lead to local thermal emergencies.

In general, servers can be more efficiently managed than presently feasible if they:

R1) Have mechanisms to put a hard limit on server energy dissipation to avoid thermal emergencies.

R2) Have a proactive mechanism to activate or deactivate virtual machines or servers or entire racks to match the offered load taking into account any energy and performance overhead for activation and deactivation.

R3) Have techniques that implement a more energy-proportional relationship between server power dissipation and the server utilization, as shown in FIG. 2B.

R4) Extend the operating region over which a server has high energy efficiency: this permits higher server energy efficiencies even at moderate load levels.

The implementation of requirements R3 and R4 lead to the situation shown in FIG. 2B. We now describe our approach to implementing these requirements in software on existing systems.

Implementing the Requirements R1 Through R4

Empirical energy dissipation models are preferably used to determine the energy consumed by a server and this estimate is used to cap the energy consumed by a server. This approach is adopted since it is not practical to use external power meters on each server to determine their energy consumption.

Empirical models for the energy dissipated by a server have been proposed in the past; the simplest of these models are based on the use of utilization data maintained by the operating system (such as core utilization, disk utilization) and is, for example, of the form: P _(server) =K ₀ +K ₁ ×U _(proc) +K ₂ ×U _(mem) +K ₃ ×U _(disk) +K ₄ ×U _(net)

Of course, other, more complex forms, may be employed.

Where the Ks are constants determined empirically and the Us refer to the utilizations of the processor (U_(proc)) 1, memory (U_(mem)), the disk(s) (U_(disk)) and the network (U_(net)). The operating system maintains and updates these utilization data. As reported in [ERK+ 08], the actual measured power and the power estimated from the above equation are quite close and typically within 10%. A recent effort extends simplistic models of this nature to regression based predictive models that predict server energy consumption on long-running jobs as a function of the core energy dissipation, L2 cache misses and ambient temperature [LGT 08]. The model of [LGT 08] is a good starting point for our efforts. We will extend this model with additional metrics obtained from hardware instrumentation counters found in typical cores as well as slightly modified system calls for network/file I/O to account for energy dissipation within network components to accurately account for remote data access and inter-process communications and I/O activity (which were ignored in the work of [LGT 08]).

To track and predict the energy consumption of servers in software, sampled measurements of the hardware instrumentation counter values and OS-maintained counters for computing utilization will be used, in manner reminiscent of our earlier work of [PKG 01]. The modified thread scheduler in contemporary Linux kernels will use these sampled measurements to guide local scheduling within a server so as to limit the server energy consumption within a sampling period to stay within the limit prescribed by the global energy/workload scheduler. In additional to the traditional DVFS adjustments, the behavior of threads within the sampling periods will be classified as CPU bound, disk bound and network bound and schedule similar threads back-to-back to avoid unnecessary changes in the DVFS settings (and avoiding the energy overhead and relatively long latencies in changing such settings). This in turn addresses Requirements R3 and R4. The modified scheduler will also react to thermal emergencies as detected by external temperature sensors (which are read and recorded periodically by the scheduler itself on scheduling events within the kernel).

Requirement R2 is implemented in the global scheduler, as described below, by keeping track of the workload trends (through monitoring of the incoming request queues at the load balancing switches) and job completion statistics. If the global scheduler sees a growth in the job arrival rate, it activates VMs/servers/racks as needed to cope with the additional workload. The overhead for such activation and deactivation, including the energy costs of moving VM contexts are accounted for in this process, and thus requirement R3 is also addressed.

Techniques for message consolidation that packs several short messages into a single message within a jumbo Ethernet frame within the network interface to amortize the flat component of per-packet overhead of network transfers may also be employed. This also addresses Requirement R3.

A different way of amortizing the scheduling overhead (including the changing of the DVFS settings of cores) exploits the characteristics of repetitive jobs. In a typical server installation, the number of such jobs is expected to be quite high. For example, repetitive jobs of the SPECweb 2006 benchmarks on a Linux platform (with Intel E5460 cores) running Apache were dynamically classified into two classes: compute bound and I/O bound, based on utilization statistics maintained by the kernel and instruction commit rate data maintained in the hardware instrumentation counters. This classification data was maintained within the Apache server. Jobs of the same class in the work queue of Apache were scheduled back-to-back wherever possible and the DVFS settings of the dual core platform were explicitly controlled. Unnecessary changes in the DVFS settings were also avoided and job wait times on the queues were limited to maintain a performance level close to that of the base case. The CPU power measurements (made with a power clamp on the power cord for the core going from the power supply to the motherboard) showed that this simply strategy reduced the core power consumption by about 11%.

For the present system, this technique can be moved to the kernel level for added efficiency, extend the classification to add memory bound jobs (jobs that trigger a high proportion of RAM activity, as evidenced by the on-chip cache miss instrumentation counter) and network bound job classes, for instance. This classification information is used to schedule jobs that match the characteristics of processor sockets with a preset independent performance or to cores within a multicore chip that permits the use of similar preset performance settings independently for each core. The preset performance settings are changed only under load increases that saturate the capacity of a core with a specific DVFS setting. This approach of exploiting pre-classed job addresses requirements R3 and R4 simultaneously.

Description of Energy Estimation Facility

The energy dissipated by a computing system can be expressed as a function of the values of some or all of the following measurable parameters (and possibly others), with the measurements made in a given time interval (see later on details of choosing the interval):

1. Overall number of instructions committed.

2. The number of various types of instructions that are committed within each core, examples of such instructions types being, but not limited to, integer instructions, memory read instructions, memory write instructions, floating point instructions, and where applicable, I/O instructions.

3. Measurements of memory activities initiated by individual cores and the microprocessor as a whole, such as, but not limited to, cache hit or miss counts, number and types of external memory accesses, memory burst sizes and number of cycles for which the processor has stalled pending a memory activity.

4. The number of instructions that are fetched but not committed within each core.

5. Measures of various activities necessary for maintaining memory and cache coherence in a multicore design.

6. The type and amount of I/O activity for the entire system. These measurements can be made locally or by external DMA controllers and made available using known techniques to the energy estimation facility.

7. Utilization data for the CPU cores, I/O devices as maintained by the operating system.

8. The “speedstep” setting of each core. It is assumed that the measurement interval is chosen such that speedstep settings remain fixed during the interval.

9. Measurements as above, but not necessarily identical to, for specialized devices within the system, such as graphics co-processors, encryption devices

Modern processors actually maintain specialized instrumentation facilities for counting many of the entities described above. The measured entities, such as, but not limited to those described above, are combined in an equation to obtain an energy dissipation for the entire system. Using feedback from sensors of actual performance data, the equation may be empirically corrected to achieve an accurate result, for a variety of system implementations. In the simplest form, the equation could be a linear one combining these measured values after weighing each measured value properly. The weights are estimated from running software and actual measurement of energy consumed by the system, to calibrate the equation. The equation can also be non-linear. The estimated energy dissipation is made available to the operating system for scheduling purposes.

Choosing the Measurement Interval:

The measurement interval can be chosen to lie in-between the consecutive instances of time at which the voltage and frequency setting of any core is changed or at regularly spaced intervals in-between these two instances of time. The measurement interval can also be chosen to lie in-between consecutive time instances at which the process scheduler is invoked. The measurement interval may be adaptive, or selected externally.

Other information that can be collected for scheduling including measured values of temperatures in various parts of the processing system, motherboard, locations in the immediate environment and other components, speed settings of fans.

Using the Estimated Energy Measurements and Temperature Measurements

Examples of the anticipated uses of the facility are, but not limited to, the following: The scheduler uses the measurements to maintain the system within an allocated energy budget for a given interval.

Software component(s) use the measured values to change the power states of the cores to stay within safe operating temperature limits, this limit being specific to one or more system components or processor components.

User-level applications can use to measured values to maintain energy dissipation targets through appropriate software and system reconfiguration.

The facility is preferably implemented as a specialized co-processor that supplies information to the software via access to special registers. This facility imports data from the various locations, as well as weights derived from the software calibration process, these weights being supplied by the software. The processing may also be implemented in hardware, or firmware defined functionality. The facility may also be implemented as a specialized kernel thread/process.

Global Energy Budget Allocation and Workload Scheduling

The global scheduler (GS) of a preferred embodiment of the system is responsible for the allocation of energy budgets for the VMs/servers/racks and the assignment of workload to the individual machines. The key requirement of the GS is that it has to be fast and scalable. The GS may be implemented on a few dedicated multicore machines which also implement the compact thermal analyzer and models. Multiple machines may be used to permit scalability; for a small server installation, it may be possible to implement all of the functions on a single multicore platform. These dedicated machines may also receive data from a variety of sources, which are optional, as shown in FIG. 1.

The GS maintains a variety of tables that record the energy/performance characteristics of each rack, its utilization statistics, and data on the environmental temperature computed from various sources. The GS also maintains a list of quality of service (QoS) requirements (guaranteed transaction rates, data delivery rates etc.) for implementing differentiated services. The GS also senses the incoming work queue sizes at the load balancing switches and uses simple workload models to predict the impact of incoming workload. The simple workload models can simply classify incoming jobs based on the request types or use more sophisticated information about pre-classified repetitive jobs. The GS schedules the workload to maximize the workload allocated to active servers/racks, assuming VM support on all nodes. This allocation uses the thermal data—obtained from the compact model as well as from thermal sensors and using service guarantees as a constraint. Cooling requirements and changes to the energy budget for the computing/storage and communication equipment for the allocated workload are also assigned based on a variety of heuristics. Some possible heuristics include (but are not limited to):

-   -   Extrapolate the thermal output of the active servers and revise         its energy budget and cooling requirement based on the updates         to number of jobs (existing plus newly-assigned) assigned to the         server.     -   Use the energy requirement characteristics of known, repetitive         jobs and the heuristic above for unclassified jobs to plan the         schedule.     -   Use the data maintained on the average job completion rate and         average energy requirement of jobs to guide the allocations.

As mentioned earlier, the GS keeps track of the job dispatch rate and the size of the incoming queues in the front-end load balancing switches to keep track of the workload trend. This trend data is used to activate or deactivate servers and racks and redirect cooling efforts as needed. The energy expended in such activation/deactivation and in migrating VMs, where necessary is accounted for in the allocations.

Alternative scheduling may also be employed, including ones that dynamically switch scheduling strategies based on the thermal conditions and current workload. As an example, if all servers are being operated in the high energy-efficiency region as shown in FIG. 2B, then it may be better to perform an allocation that balances the load across the racks to avoid the formation of hot spots in the server room.

The GS has similarities with data center configuration systems and mangers from several vendors (e.g., IBM's Tivoli suite) [IBM 08a, IBM 08b]. However, the present system differs from these schedulers in at least the way server energy dissipation estimates are made at a finer granularity, in making use of a thermal model to predict and cope with thermal conditions, and in using dynamic cooling systems.

Control Systems Issues

The present technique is essentially a control system that employs reactive as well as proactive actuations to meet the goal of improving the overall energy efficiency of a data center. As such, it has to be scalable, stable and provide appropriate sense-and-actuate latencies. Another important requirement of the system is that the various control elements should act in a synchronized and coordinated manner, avoiding “power struggles” [RRT+ 08], where one control loop fights against another inadvertently.

On the control elements at the computing side, these control system requirements are met by a using a hierarchical implementation that uses independent control elements at each level and by using a progressive top-down approach to dictate the energy/performance goals of one level to be explicitly dictated by the control system at the immediately upper level. The hierarchical control mechanisms of the activities within a computing system also ensures its scalability: separate control loops are used to ensure the energy budgets at the rack level and at the level of individual servers within the rack are monitored and managed separately. For large data centers, another level can be added to make the system more scalable, based on the allocation and control of the energy budgets for a set of neighboring racks.

The control of the computing equipment is based on the notion of update intervals within a sampling period, with sensor and model outputs collected at the end of each update period. At the end of a sampling period, the values of respective sensor and model data output are averaged, and control decisions taken at the end of a sampling period based on these average values, as introduced in [PKG 01]. This approach smoothes out the impact of burst activities that are inevitable within a sampling interval and enables a stable control system for the computing elements.

Hardware Overview

FIG. 3 (see U.S. Pat. No. 7,702,660, issued to Chan, expressly incorporated herein by reference), shows a block diagram that illustrates a computer system 400 upon which an embodiment of the invention may be implemented. Computer system 400 includes a bus 402 or other communication mechanism for communicating information, and a processor 404 coupled with bus 402 for processing information. Computer system 400 also includes a main memory 406, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 402 for storing information and instructions to be executed by processor 404. Main memory 406 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 404. Computer system 400 further includes a read only memory (ROM) 408 or other static storage device coupled to bus 402 for storing static information and instructions for processor 404. A storage device 410, such as a magnetic disk or optical disk, is provided and coupled to bus 402 for storing information and instructions.

Computer system 400 may be coupled via bus 402 to a display 412, such as a cathode ray tube (CRT) or liquid crystal flat panel display, for displaying information to a computer user. An input device 414, including alphanumeric and other keys, is coupled to bus 402 for communicating information and command selections to processor 404. Another type of user input device is cursor control 416, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 404 and for controlling cursor movement on display 412. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.

The invention is related to the use of computer system 400 for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 400 in response to processor 404 executing one or more sequences of one or more instructions contained in main memory 406. Such instructions may be read into main memory 406 from another machine-readable medium, such as storage device 410. Execution of the sequences of instructions contained in main memory 406 causes processor 404 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software.

The term “machine-readable medium” as used herein refers to any medium that participates in providing data that causes a machine to operation in a specific fashion. In an embodiment implemented using computer system 400, various machine-readable media are involved, for example, in providing instructions to processor 404 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 410. Volatile media includes dynamic memory, such as main memory 406. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 402. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. All such media must be tangible to enable the instructions carried by the media to be detected by a physical mechanism that reads the instructions into a machine.

Common forms of machine-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

Various forms of machine-readable media may be involved in carrying one or more sequences of one or more instructions to processor 404 for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 400 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 402. Bus 402 carries the data to main memory 406, from which processor 404 retrieves and executes the instructions. The instructions received by main memory 406 may optionally be stored on storage device 410 either before or after execution by processor 404.

Computer system 400 also includes a communication interface 418 coupled to bus 402. Communication interface 418 provides a two-way data communication coupling to a network link 420 that is connected to a local network 422. For example, communication interface 418 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 418 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 418 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

Network link 420 typically provides data communication through one or more networks to other data devices. For example, network link 420 may provide a connection through local network 422 to a host computer 424 or to data equipment operated by an Internet Service Provider (ISP) 426. ISP 426 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet” 428. Local network 422 and Internet 428 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 420 and through communication interface 418, which carry the digital data to and from computer system 400, are exemplary forms of carrier waves transporting the information.

Computer system 400 can send messages and receive data, including program code, through the network(s), network link 420 and communication interface 418. In the Internet example, a server 430 might transmit a requested code for an application program through Internet 428, ISP 426, local network 422 and communication interface 418.

The received code may be executed by processor 404 as it is received, and/or stored in storage device 410, or other non-volatile storage for later execution. In this manner, computer system 400 may obtain application code in the form of a carrier wave.

In this description, several preferred embodiments were discussed. Persons skilled in the art will, undoubtedly, have other ideas as to how the systems and methods described herein may be used. It is understood that this broad invention is not limited to the embodiments discussed herein. Rather, the invention is limited only by the following claims.

REFERENCES Each of which is Expressly Incorporated by Reference

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What is claimed is:
 1. A method of scheduling tasks, comprising: receiving data conveying information relating to a plurality of prospective tasks in a processing queue; computing, with at least one automated processor, estimates of prospective energy dissipation over time for executing a load comprising the plurality of tasks, based on at least the received data and predetermined calibration parameters for associating respective tasks with a related energy dissipation over time; and selectively scheduling the plurality of prospective tasks for processing by at least one resource which each has a respective temperature determined by different factors, based on at least the computed estimate of energy dissipation affected by the scheduled tasks, and at least a predicted load-dependent temperature associated with each at least one resource over time.
 2. The method of claim 1, wherein the scheduling of the plurality of prospective tasks is further dependent on a predicted system load and the plurality of prospective tasks are distributed unevenly to the at least one resource.
 3. The method of claim 1, wherein the scheduling of the plurality of prospective tasks is dependent on a predicted load-dependent temperature associated with each of at least two resources over time.
 4. The method of claim 1, wherein the scheduling of the plurality of prospective tasks is based on at least the computed estimate of energy dissipation affected by the scheduled tasks, a predicted system load, and a predicted load-dependent temperature associated with each at least two resources over time.
 5. The method of claim 1, further comprising coordinating a control of a dynamically controllable cooling system with the scheduling, the dynamically controllable cooling system being effective for varying a relationship between an aggregate energy dissipation over time and temperature of the at least one resource.
 6. The method of claim 1, wherein the scheduling comprises reordering a plurality of elements in the processing queue.
 7. The method of claim 1, further comprising selectively scheduling the plurality of prospective tasks for processing by at least one resource to meet at least one service guaranty.
 8. The method of claim 1, further comprising recruiting at least one inactive resource to become available for executing tasks in dependence on a predicted temperature associated with at least one other resource, such that the predicted load dependent temperature associated with the at least one other resource does not exceed a maximum operating temperature.
 9. An apparatus for scheduling tasks, comprising: a processing queue configured to receive data conveying information relating to a plurality of prospective tasks; at least one automated processor, configured to compute estimates of prospective energy dissipation over time for executing a load comprising the plurality of tasks based on at least the received data and predetermined calibration parameters for associating respective tasks with a related energy dissipation over time; and a scheduler configured to schedule the plurality of prospective tasks for processing by at least one resource having a respective temperature determined by different factors, based on the computed estimate of energy dissipation affected by the scheduled tasks, and a predicted load-dependent temperature associated with each of the at least one resource over time.
 10. The apparatus of claim 9, wherein the scheduler is configured to schedule the plurality of prospective tasks in dependence on at least a predicted system load, such that the plurality of prospective tasks are distributed unevenly to the at least one resource.
 11. The apparatus of claim 9, wherein the scheduler is configured to schedule the plurality of prospective tasks in dependence on a predicted load-dependent temperature associated with each of at least two resources over time.
 12. The apparatus of claim 9, wherein the scheduler is configured to schedule the plurality of prospective tasks in dependence on at least the computed estimate of energy dissipation affected by the scheduled tasks, a predicted system load, and a predicted load-dependent temperature associated with each of at least two resources over time.
 13. The apparatus of claim 9, further comprising a control for dynamically controlling a cooling system, configured to coordinate operation of the dynamically controllable cooling system with the schedule of the plurality of tasks, and thereby selectively influence both an allocation of the plurality of prospective tasks to a respective at least one resource and a cooling of the respective at least one resource.
 14. The apparatus of claim 9, wherein each respective task has a classification based on of a plurality of different classifications based on usage characteristics of the at least one resource, and the scheduler is configured to selectively schedule respective tasks having the same classification back to back.
 15. The apparatus of claim 9, wherein the scheduler is further configured to selectively schedule the plurality of prospective tasks for processing by at least one resource to meet at least one service guaranty.
 16. The apparatus of claim 9, further comprising at least one inactive resource, the scheduler being further configured to activate the at least one inactive resource to become available for executing tasks in dependence on a predicted temperature associated with at least one active resource, such that the predicted load dependent temperature associated with the at least one active resource does not exceed a maximum operating temperature.
 17. A method of scheduling tasks, comprising: maintaining a queue of tasks to be prospectively processed by a system having at least two processing resources; estimating an energy dissipation over time for processing the respective tasks in the queue based on previous energy dissipation for corresponding tasks; and reordering tasks in the queue of tasks for selective dispatch to each of the at least two processing resources, based on at least the estimated energy dissipation over time affected by the scheduled tasks, and a predicted load-dependent temperature associated with each resource over time.
 18. The method of claim 17, wherein the allocating is further dependent on a predicted system load and at least one quality of service constraint.
 19. The method of claim 17, further comprising coordinating a control of a dynamically controllable cooling system with the allocating, the dynamically controllable cooling system being effective for varying a relationship between an aggregate energy dissipation over time and temperature of the at least one resource.
 20. The method of claim 17, further comprising activating at least one additional resource to process tasks in dependence on at least the estimated energy dissipation over time affected by the scheduled tasks, such that the predicted load dependent temperature associated with the at least two active resources does not exceed a respective maximum operating temperature.
 21. The method of claim 17, wherein each respective task has a classification based on of a plurality of different classifications based on usage characteristics of the at least one resource, and the reordering of tasks in the queue of tasks for dispatch comprises reordering the tasks for dispatch to each least two alternate resources the scheduler to selectively schedule respective tasks having the same classification back to back on the same respective at least one resource. 